1. Field of the Invention
The present invention relates in general to a navigation system, more particularly, to a method for correcting position error in a navigation system which utilizes a map matching (MM) in order to provide an accurate information on a current position of a moving object by correcting position errors in consideration of position measurements of a mobile object including vehicles, airplanes and ships, and the shape of a road (i.e. link) displayed on a digital map.
2. Discussion of the Background Art
An essential element of navigation systems is the determination of a current position of a moving object (e.g. vehicles) within an allowable range of error, and providing such information to a driver of the moving object continuously in real time mode because the moving object's current position is used for marking a current position of the mobile object, providing route guidance information, and perceiving the surrounding road environment.
Providing incorrect current positions of the moving object or providing position information discontinuously may cause serious damages on the safety of an operator of the moving object, so the determination of current positions of the moving object is very important.
Navigation systems do not always provide accurate information on a current position of the moving object. That is, the error range of sensors such as a gyro (or gyroscope) or electronic compass built in the moving object could be great, depending on rotation direction of the moving object. Also, when the moving object stops, its heading may be deflected in one direction or be changed, making position errors on heading of the moving object. Moreover, measurement of traveling distance of the moving object using a wheel sensor can be erroneous also because the wheel sensor is very sensitive to external factors like air pressure of wheels/tires and road surface conditions.
Another material for informing the current position of the moving object is a digital map that is drawn over a central line (i.e. a solid yellow line). Many times, however, operators of moving objects do not drive along an intersection or central line on a road, and as a result, the digital map itself have errors thereon. In addition, errors are also made when inputting data in the digital map, marking the position of the moving object at an incorrect spot. Of course, these errors should be absolutely corrected.
As an attempt to solve the above problems, Korean Patent No. 216535 (Applin. No. 1997-24564) disclosed a method for measuring a current position of a moving object.
According to the above disclosure, a dead reckoning system (DR) using information collected from a heading and distance sensors attached to a moving object provides a current position of the moving object, which is designated as a first position, and by a map matching method using position matching, the first position is corrected or maneuvered onto a digital map, and this corrected position is designated as a second position. Further, the current position of the moving object is measured using a signal provided from GPS, and the current position at this time is designated as a third position. Similar to before, applying the map matching method using position matching, the third position is corrected or maneuvered onto the digital map, and this corrected position is designated as a fourth position. With these first through fourth positions, short distance driving patterns of an operator of the moving object are obtained. Afterwards, similarities between each of the short distance driving patterns at the first through fourth positions and road patterns on the digital map are compared with each other, and from the comparison, a position having the highest degree of similarity is finally selected as the current position of the moving object.
However, the above technique did not really use the result of map matching to trace the moving object. Instead, combinations of the DR and GPS systems, namely signals from sensors and GPS, were mainly used to match the current position of the moving object being measured with the digital map. Therefore, it was rather difficult to do perfect matching of the position of the moving object with the digital map.
For example, depending on geographical features in an area where the moving object is at, the roads are sometimes straight but sometimes they are curved. The position of the moving object on a straight road can be easily matched with the digital map, but on a curved road, accurate positioning is very difficult.